1. Field of the Invention
The present disclosure relates to a density detection technique of an image forming apparatus using an electrophotographic method, for example, a copier, printer, or facsimile.
2. Description of the Related Art
At present, as image output terminals, image forming apparatuses, such as printers, have become widespread owing to the development of computer network technologies, and in recent years, the demand for improving the stability of the image quality of images formed by image forming apparatuses has heightened. Relating in particular to the density reproducibility of images, a sophisticated stability is being sought that does not vary with changes in the installation environment of the image forming apparatus, or temporal changes, or depends on characteristic difference of each apparatus. However, the density of an image formed by an image forming apparatus varies based on changes in each driving member or image forming member owing to continuous use, or fluctuations in temperature within the apparatus, or the like; therefore, it is not possible to meet the demanded high level of stability by keeping the initial settings. Therefore, it is common to perform calibration (hereafter, called “density control”) in order to maintain optimum image density.
In density control, first a developer image for test purposes (hereafter, called a “test pattern”) is formed on a cyclically moving member, for example, a photosensitive member, an intermediate transfer member, or a feeding belt, and a physical quantity that correlates with the position of the test pattern and an amount of developer is measured using a sensor. Using these measurement results and the conditions at the time of forming a test pattern, each of the control targets, such as charging bias, developing bias, and exposure amount, are then controlled so that the actual image density at printing is suitable.
Moreover, in order to detect a test pattern with a sensor, the test pattern must be made larger than the spot diameter of the light irradiated by the sensor. On the other hand, the developer consumed in density control is considered wasted consumption on the part of the apparatus by the user, and must be reduced as much as possible.
Japanese Patent Laid-Open No. 2005-241933 discloses an optical sensor with high precision of irradiation angle even with a small aperture on the light emitting side, and with high accuracy of reading of the test pattern regardless of variation in irradiated area owing to manufacturing variation. Density can be detected accurately with this optical sensor, even for a relatively small test pattern, because the spatial resolution and detection accuracy can be increased together. Japanese Patent Laid-Open No. 2009-134037 discloses a density control method that uses a sensor having two photodiodes which respectively receive specular reflected light and diffuse reflected light from a test pattern. Japanese Patent Laid-Open No. 2009-134037 indicates that the size of the test pattern for density control depends on the spot diameter of the diffuse reflected light, which is larger in size than that of the specular reflected light.
With respect to image forming apparatuses, further reduction of the amount of developer consumed in order to form a test pattern for density control purposes is being sought. At the same time, a high level of image density reproducibility is desired. Therefore, an effective detection method for performing accurate density control even for small test patterns is required. In order to suppress the effects of variation in the surface conditions of the target objects for which a test pattern is to be formed, even in cases where a sensor with high spatial resolution and high detection precision is used, it is necessary to average a degree of the measurement results. Further, it is necessary to calculate the amount of diffuse reflected light in order to accurately calculate the amount of specular reflected light; therefore, it is necessary to detect the amount of specular reflected light and diffuse reflected light from the same area. Here, the spot diameter of diffuse reflected light is larger than the spot diameter of specular reflected light; therefore, when detecting the amount of specular reflected light and amount of diffuse reflected light from the same area of the target object, the spot diameter of the diffuse reflected light is a factor in determining the size of the test pattern.